1. Field of the Invention
This invention relates generally to wind turbines, and more particularly to an improved turbine with mechanical means for controlling constant rotation.
2. Description of Related Art
For many decades, oil, gas and coal have been the world's dominate power sources. However, as global warming, ozone depletion, water pollution and many other negative consequences pose more and more of a threat to the planet, we have begun to question our reliance on fossil fuels and search for more sustainable, less damaging alternative power sources. Wind power seems to be a perfect solution, as harnessing the power of the wind does not create any toxic by-products nor does it spew chemicals into the air and water. Most importantly, however, there is an abundant, virtually unlimited supply of wind, as just one percent of the Earth's winds could theoretically meet the entire world's energy needs.
Originally, wind power was only used by farmers and ranchers who where outside of the power grid altogether. However, tax incentives offered in the 1980's led to the introduction of wind power on a somewhat larger scale. Today, there are nearly 17,000 turbines in the United States, and Europe has almost 2,000 megawatts of generating capacity. Although this still accounts for less than 1 percent of the nation's electricity, the use of wind power is rapidly increasing, and the U.S. Dept. of Energy predicts a sixfold increase in the nation's wind-energy use during the next 15 years.
This sudden move toward wind power as a viable power source alternative is due to advanced turbine technology that has dramatically increased the efficiency of turbines and decreased their operational costs to the point that wind power is now economically competitive with fossil fuels and nuclear energy. In fact, it is the goal of the National Renewable Energy Lab to develop wind turbines by the year 2000 that will generate electricity at a cost of four cents per kilowatt-hour as sites with only moderate wind speeds, thus making wind power slightly less expensive than both oil and gas.
One key development that lead to increased efficiency in turbine technology was the creation of airfoils designed specifically for wind turbines. Wind blowing past a turbine does not push the blades, but rather the air passing over the blade's upper surface travels farther than air crossing the underside, thus resulting in a pressure difference that creates lift. As lift drives the blades forward, they turn a drive shaft connected to a generator. Once airfoils based on these principles were designed and installed in conventional turbines, energy capture increased by approximately 30%.
As technology has increased, the size of the turbines has also significantly increased, as the way to capture more energy at lower wind speeds is to use longer blades and taller towers. The largest turbine currently in use is the Z-40 turbine manufactured by Zond Systems of Tehachapi, Calif. This turbine has a three-blade rotor that measures 40 meters in diameter. The rotor is designed to turn at a constant speed so that the turbine consistently cranks out electric current at a 60 Hz frequency. A generator of the device is designed to create drag when needed so as to keep the blades rotating at the correct speed. At sites where average wind speed is 15 to 16 mph, the Z-40 generates electricity for approximately five to six cents per kilowatt-hour.
Unfortunately, while a turbine's power output may increase in proportion to its size, the Z-40 faces many problems due to its extremely large size. First of all, a single turbine costs nearly $500,000 to produce. The blades, rotor hub and nacelle of the device alone weigh approximately 55,000 pounds, the size and weight of the device requiring special shipping procedures to transport it to its on-site location. Even further, because its blades are so long, its pitch-control system cannot immediately respond to wind gusts. To overcome this problem, ailerons that are much shorter than the entire blades are installed on the device so that they can respond faster when wind speed becomes dangerous. However, the use of such ailerons makes the device susceptible to icing and thus makes the turbine unsuitable for use in cold climates.
The problems and cost considerations involved with turbines of such a large size will likely prevent turbines from growing much larger than the Z-40. Instead, technology has shifted from extremely large, heavy-duty turbines to smaller, lightweight devices designed with an emphasis on aerodynamics rather than size. The AWT-26 designed by Advanced Wind Turbines of Seattle is an example of one such turbine apparatus that is currently in use. This device has a rotor diameter of 26.2 meters, has only two blades and has a weight less than one third of that of the Z-40. The turbine spins at 57 rpm, which is about twice as fast as the Z-40. Unlike some three-blade designs, the two blades of this device have a fixed pitch because they don't pivot at the hub. Instead, the blades of the AWT-26 flex as wind speed increases.
Although the AWT-26 improves upon the Z-40 in that it is significantly smaller and less expensive to manufacture, its two blade configuration makes it more susceptible to premature fatigue and damage because wind forces acting on the blades aren't always equal. In addition, with this two-blade configuration, the blades must pass into the tower's "wind shadow" on every rotation. Not only does this fatigue the blades, but it also produces a great deal of noise with each rotation.
Another similar turbine is the Northwind 250 created by New World Power Technology Co. of Moretown, Vt. This device is a 250-kilowatt turbine that also has only two blades. The two fiberglass blades are constructed as a single piece tip to tip with an aluminum clamp fitting over the center of the rotor and a rubber bearing separating the aluminum and fiberglass. This construction effectively eliminates the need for a flange at the root of each blade, which results in approximately 25% savings on the cost of the blade. The Northwind 250 overcomes the noise problem caused by the blades passing into the tower's "wind shadow" on each rotation by yawing the rotor into the wind. Rather than moving the entire blade, the device has a small aileron near the tip of the blades' trailing edge. Unlike the aileron on the Z-40 which is used only for braking, the aileron on this device is simply adjusted to control the pitch of the blade while it is moving. As the aileron is deployed, the lift goes down, the drag goes up and the noise is reduced.
Like the AWT-26, the Northwind 250 uses aerodynamic principles to achieve more efficient operation of the turbine. However, in order to overcome the excessive noise problem associated with a two blade design, this device requires a relatively expensive yaw drive. The Northwind 250, AWT-26 and the Z-40 are all constant-speed machines that must rotate at a fixed speed and thus are incapable of operating during a variety of different wind conditions. There are several different ways by which to monitor the speed of the turbine, but ultimately, when the blade's angle of attack becomes so steep that the airflow around the blades is too turbulent to produce lift, the turbine stalls. The Z-40, for example, shuts down at wind speeds above approximately 65 mph, during light wind conditions and during power outages by feathering its blades and turning them parallel to the wind. The blades of the AWT-26, on the other hand, flex as wind speed increases until they eventually stall. Because the airfoil has a different shape at the blade's root than at its tip, the root section stalls earlier than the tip. While this lets the blade extract the maximum amount of power from the wind before it completely stalls, once the blades stall power can no longer be harnessed.
The KVS-33 turbine having a rotor diameter of 33 meters is produced by Kenetech Windpower of San Francisco and is the first variable-speed machine to be widely accepted by utilities. It is more efficient than comparable constant-speed machines because its rotor speeds up or slows down to match shifts in wind velocity, thus allowing it to generate power in virtually all wind conditions. The device has two generators that produce alternating current with a range of frequencies, rather than the precise 60 Hz demanded by the US power grid. A power-control system converts the fluctuating alternating current to direct current and then inverts it back into 60 Hz alternating current. The turbine's ability to accommodate all wind conditions makes it one of the most efficient turbines currently available. Each individual turbine includes a controller that monitors wind gusts and other conditions and automatically adjusts the turbine according to the monitored conditions. The individual turbine controllers then report to a central computer that keeps track of utility load requirements.
While this device is significantly more effective than prior art constant-speed turbines, its heavy reliance on electronic controls to achieve maximum efficiency make the device susceptible to significant damage in the event of a power outage or malfunction of the controls. Even further, the sophisticated electronic controls installed into each individual turbine significantly add to the cost of manufacturing and operating the device.
Thus there is a clear need for an improved turbine apparatus that can function in a wider range of wind conditions without relying on excessive electronical controls. Such a device would automatically adjust to remain at a constant rotational speed despite changes in wind velocity. The present invention fulfills these needs and provides further related advantages as described in the following summary.